The dynamics that are involved when a golf club properly hits a golf ball are extremely complicated. Due to the particular configuration of a golf club, both bending and torsional forces are directly transferred from the club head to the club shaft whenever a golf ball is hit. The interaction of these transferred forces with the impact force on the ball is important for at least two reasons. On the one hand, for purposes of control, it is important that the proper indications of the impact force between the golf ball and the club head be transferred through the golf club shaft to the golfer. This gives the golfer the so-called "feel" of the swing. On the other hand, for purposes of power, it is important that the golf ball receive as much of the impact force as possible. For both purposes, it is also important that the club shaft be able to withstand the forces which are generated.
It is not an all together uncommon occurrence for a club head to break or snap off from the club shaft, or for the club shaft to otherwise become permanently deformed, when the club head contacts a golf ball, or some other object, during a golf swing. Based on empirical evidence, it happens most often that this unwanted event is the result of bending forces which are generated on the club shaft as the golf ball is being hit. To illustrate this point, refer for the moment to the drawings and, specifically, to FIGS. 4, 5A and 5B.
During the swing of a golf club, forces are generated on the club even before there is impact with the golf ball (see FIG. 4). From a structural standpoint, these forces are not overly significant. Upon impact with the golf ball, however, forces are generated on the golf club which are, structurally, very significant. As shown in FIG. 5A, during the impact of the club head with a golf ball, the impact force causes the golf shaft to bend. Specifically, the golf shaft bends in a manner which generates a tension force, T.sub.f, on the forward edge of the shaft, and a compression force, C.sub.r, on the rear edge of the shaft. Immediately after impact, the force distribution is reversed and the shaft bends in the opposite direction. As shown in FIG. 5B, once the golf ball has separated from the club head, the shaft bends in a manner which generates a compression force, C.sub.f, on the forward edge of the shaft, and a tension force, T.sub.r, on the rear edge of the shaft. It is known that the action of this force reversal will continue briefly and can either cause a permanent deformation of the shaft or cause the club head to actually snap off the club shaft. To avoid these unwanted results, and to provide a meaningful and useful response to the golfer, the manufacturer of a golf club shaft must address several issues. Specifically, the materials, the characteristics of the materials, and the configuration of the materials that are used for the manufacture of the golf shaft all become important considerations. More specifically, for the dynamic conditions confronted during the swing of a golf club, material elasticity is a primary concern.
Several aspects of material elasticity need to be considered when designing a golf club shaft. To begin, by definition, the elasticity of a material is the tendency of the material to return to its original size or shape, after it has been stretched, compressed, or deformed. While all materials have some elasticity, they also have limits. Specifically, the elastic limit of a material is the limiting value of the deforming force beyond which the material does not return to its original shape or dimensions when the force is removed. Not surprisingly, it happens that different materials have different elastic limits. Furthermore, the responsive behavior of different materials within their respective elastic ranges will also be different.
An indicator of the different responses of materials in their elastic ranges is known as the material's modulus of elasticity. Again, by definition, the modulus of elasticity of a material is the ratio of stress to strain that is experienced by a material in its elastic range. More specifically, the modulus of elasticity is the ratio of the stress that is caused in a material by the action of deforming forces to the strain or change in dimensions or shape of the material during deformation. Insofar as the manufacture of a golf club shaft is concerned, the modulus of elasticity is an important consideration for determining whether the particular material will be able to handle the forces which are generated during a golf swing. Additionally, when different materials are used in the manufacture of a golf club shaft, the location and configuration of these materials, as well as the interactive and cooperative effects of their respective moduli of elasticity will be important considerations.
In light of the above, it is an object of the present invention to provide a golf club shaft which has material characteristics that will withstand the repeated impact forces to which the shaft is routinely subjected during a game of golf. It is another object of the present invention to provide a golf club shaft which is able to transfer impact forces to the golfer in an informative and meaningful manner. Still another object of the present invention is to provide a golf club shaft which focuses and concentrates the impact forces between a club head and a golf ball onto the golf ball for improved performance. Yet another object of the present invention is to provide a method for manufacturing a golf club shaft which is relatively easy to perform and comparatively cost effective.